Agent skill

openapi-to-typescript

Converts OpenAPI 3.0 JSON/YAML to TypeScript interfaces and type guards. This skill should be used when the user asks to generate types from OpenAPI, convert schema to TS, create API interfaces, or generate TypeScript types from an API specification.

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npx add-skill https://github.com/davila7/claude-code-templates/tree/main/cli-tool/components/skills/development/openapi-to-typescript

SKILL.md

OpenAPI to TypeScript

Converts OpenAPI 3.0 specifications to TypeScript interfaces and type guards.

Input: OpenAPI file (JSON or YAML) Output: TypeScript file with interfaces and type guards

When to Use

  • "generate types from openapi"
  • "convert openapi to typescript"
  • "create API interfaces"
  • "generate types from spec"

Workflow

  1. Request the OpenAPI file path (if not provided)
  2. Read and validate the file (must be OpenAPI 3.0.x)
  3. Extract schemas from components/schemas
  4. Extract endpoints from paths (request/response types)
  5. Generate TypeScript (interfaces + type guards)
  6. Ask where to save (default: types/api.ts in current directory)
  7. Write the file

OpenAPI Validation

Check before processing:

- Field "openapi" must exist and start with "3.0"
- Field "paths" must exist
- Field "components.schemas" must exist (if there are types)

If invalid, report the error and stop.

Type Mapping

Primitives

OpenAPI TypeScript
string string
number number
integer number
boolean boolean
null null

Format Modifiers

Format TypeScript
uuid string (comment UUID)
date string (comment date)
date-time string (comment ISO)
email string (comment email)
uri string (comment URI)

Complex Types

Object:

typescript
// OpenAPI: type: object, properties: {id, name}, required: [id]
interface Example {
  id: string;      // required: no ?
  name?: string;   // optional: with ?
}

Array:

typescript
// OpenAPI: type: array, items: {type: string}
type Names = string[];

Enum:

typescript
// OpenAPI: type: string, enum: [active, draft]
type Status = "active" | "draft";

oneOf (Union):

typescript
// OpenAPI: oneOf: [{$ref: Cat}, {$ref: Dog}]
type Pet = Cat | Dog;

allOf (Intersection/Extends):

typescript
// OpenAPI: allOf: [{$ref: Base}, {type: object, properties: ...}]
interface Extended extends Base {
  extraField: string;
}

Code Generation

File Header

typescript
/**
 * Auto-generated from: {source_file}
 * Generated at: {timestamp}
 *
 * DO NOT EDIT MANUALLY - Regenerate from OpenAPI schema
 */

Interfaces (from components/schemas)

For each schema in components/schemas:

typescript
export interface Product {
  /** Product unique identifier */
  id: string;

  /** Product title */
  title: string;

  /** Product price */
  price: number;

  /** Created timestamp */
  created_at?: string;
}
  • Use OpenAPI description as JSDoc
  • Fields in required[] have no ?
  • Fields outside required[] have ?

Request/Response Types (from paths)

For each endpoint in paths:

typescript
// GET /products - query params
export interface GetProductsRequest {
  page?: number;
  limit?: number;
}

// GET /products - response 200
export type GetProductsResponse = ProductList;

// POST /products - request body
export interface CreateProductRequest {
  title: string;
  price: number;
}

// POST /products - response 201
export type CreateProductResponse = Product;

Naming convention:

  • {Method}{Path}Request for params/body
  • {Method}{Path}Response for response

Type Guards

For each main interface, generate a type guard:

typescript
export function isProduct(value: unknown): value is Product {
  return (
    typeof value === 'object' &&
    value !== null &&
    'id' in value &&
    typeof (value as any).id === 'string' &&
    'title' in value &&
    typeof (value as any).title === 'string' &&
    'price' in value &&
    typeof (value as any).price === 'number'
  );
}

Type guard rules:

  • Check typeof value === 'object' && value !== null
  • For each required field: check 'field' in value
  • For primitive fields: check typeof
  • For arrays: check Array.isArray()
  • For enums: check .includes()

Error Type (always include)

typescript
export interface ApiError {
  status: number;
  error: string;
  detail?: string;
}

export function isApiError(value: unknown): value is ApiError {
  return (
    typeof value === 'object' &&
    value !== null &&
    'status' in value &&
    typeof (value as any).status === 'number' &&
    'error' in value &&
    typeof (value as any).error === 'string'
  );
}

$ref Resolution

When encountering {"$ref": "#/components/schemas/Product"}:

  1. Extract the schema name (Product)
  2. Use the type directly (don't resolve inline)
typescript
// OpenAPI: items: {$ref: "#/components/schemas/Product"}
// TypeScript:
items: Product[]  // reference, not inline

Complete Example

Input (OpenAPI):

json
{
  "openapi": "3.0.0",
  "components": {
    "schemas": {
      "User": {
        "type": "object",
        "properties": {
          "id": {"type": "string", "format": "uuid"},
          "email": {"type": "string", "format": "email"},
          "role": {"type": "string", "enum": ["admin", "user"]}
        },
        "required": ["id", "email", "role"]
      }
    }
  },
  "paths": {
    "/users/{id}": {
      "get": {
        "parameters": [{"name": "id", "in": "path", "required": true}],
        "responses": {
          "200": {
            "content": {
              "application/json": {
                "schema": {"$ref": "#/components/schemas/User"}
              }
            }
          }
        }
      }
    }
  }
}

Output (TypeScript):

typescript
/**
 * Auto-generated from: api.openapi.json
 * Generated at: 2025-01-15T10:30:00Z
 *
 * DO NOT EDIT MANUALLY - Regenerate from OpenAPI schema
 */

// ============================================================================
// Types
// ============================================================================

export type UserRole = "admin" | "user";

export interface User {
  /** UUID */
  id: string;

  /** Email */
  email: string;

  role: UserRole;
}

// ============================================================================
// Request/Response Types
// ============================================================================

export interface GetUserByIdRequest {
  id: string;
}

export type GetUserByIdResponse = User;

// ============================================================================
// Type Guards
// ============================================================================

export function isUser(value: unknown): value is User {
  return (
    typeof value === 'object' &&
    value !== null &&
    'id' in value &&
    typeof (value as any).id === 'string' &&
    'email' in value &&
    typeof (value as any).email === 'string' &&
    'role' in value &&
    ['admin', 'user'].includes((value as any).role)
  );
}

// ============================================================================
// Error Types
// ============================================================================

export interface ApiError {
  status: number;
  error: string;
  detail?: string;
}

export function isApiError(value: unknown): value is ApiError {
  return (
    typeof value === 'object' &&
    value !== null &&
    'status' in value &&
    typeof (value as any).status === 'number' &&
    'error' in value &&
    typeof (value as any).error === 'string'
  );
}

Common Errors

Error Action
OpenAPI version != 3.0.x Report that only 3.0 is supported
$ref not found List missing refs
Unknown type Use unknown and warn
Circular reference Use type alias with lazy reference

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